Imagine you've been brought up believing that you can't know God. That God is angry and remote - and as terrifying as he/she is mystifying. Suddenly you meet a guy who knows both what God is like (because he has revealed himself) and what God wants (because God speaks to him). It's huge...! It's what you've always wanted.
The shock that there is an all powerful God who can be known and who doesn't leave you guessing as to what pleases him is doubled: you find that the grumpy racist bigot, who knows all, this has decided to stick two fingers up at the heavens and go his own way. He takes it so much for granted that he doesn't feel at all compelled to actually do what God has told him to do.
The things that Jonah takes for granted (that God can be known, that he reveals himself, that he is faithful and reliable) bring pagan sailors to their knees.
Seeing their reaction, Jonah begins to recognise how great God is and how blessed he is: in prayer he contrasts the hopelessness of pagan idolatry with the knowledge of God's steadfast love (Jonah 2:8).
When someone from a non-Christian background comes to faith in Christ, typically they seem to grow - at least in the early years - much faster than someone who's been brought up in a Christian family and finally accepted it. (The puritans used to say that the same sun that melts the ice hardens the clay - and there is a real risk, particularly with teenagers, that forcing them to come to church can result in hardening their hearts against the gospel). Their lives change rapidly; they don't read the bible assuming it to (basically) validate the way they live: they read it expecting it to convict and change them. The word is fresh and, typically, causes them respond with repentance and obedience.
All of which is a great challenge to Christians who are accustomed to a more laid back, more English, approach to their faith. Whether we're talking about pagan sailors in the Mediterranean or Roman centurions who've suddenly realised that the man they've just crucified truly is the Son of God, or a Chinese student who's suddenly found out that there is a God who loves her, or guys in Paulsgrove who've found that what they've been searching is satisfied by a relationship with Jesus, the impact of the gospel when it is fresh to the soul rebukes our laxness and provokes us to repent.